29.不同的二叉搜索樹 II

題目描述：

給定一個整數 n，生成所有由 1 … n 爲節點所組成的二叉搜索樹。
示例:

輸入: 3
輸出:
[
  [1,null,3,2],
  [3,2,null,1],
  [3,1,null,null,2],
  [2,1,3],
  [1,null,2,null,3]
]
解釋:
以上的輸出對應以下 5 種不同結構的二叉搜索樹：

   1         3     3      2      1
    \       /     /      / \      \
     3     2     1      1   3      2
    /     /       \                 \
   2     1         2                 3

代碼實現：

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {number} n
 * @return {TreeNode[]}
 */
var generateTrees = function(n) {
    if (n === 0) return []
    if (n === 1) return [new TreeNode(1)]
    var res = [new TreeNode(1), new TreeNode(2)]
    res[0].right = new TreeNode(2)
    res[1].left = new TreeNode(1)
    var num = 3
    while (num <= n) {
        var cache = []
        res.forEach(root => {
            var newRoot = new TreeNode(num)
            newRoot.left = root
            cache.push(newRoot)
            var node = root
            while (node) {
                newN = new TreeNode(num)
                var newNode = new TreeNode(node.val)
                newNode.left = node.left
                newNode.right = newN
                newN.left = node.right
                var newRoot = node === root ? newNode : copyTree(root, newNode)
                cache.push(newRoot)
                node = node.right
            }
        })
        res = cache
        num++
    }
    return res

    function copyTree(root, target) {
        var res = new TreeNode(root.val), parent = res
        while (root.right) {
            var node = new TreeNode(root.right.val)
            if (root.right.val === target.val) {
                parent.right = target
                parent.left = root.left
                break
            }
            parent.right = node
            parent.left = root.left
            parent = node
            root = root.right
        }
        return res
    }
};